The present invention relates to semiconductor materials, and, more particularly, to the annealing and doping of mercury cadmium telluride and related materials.
Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as photosensitive semiconductors for infrared radiation. Indeed, Hg.sub..8 Cd.sub..2 Te has a bandgap of about 0.1 eV which corresponds to a photon wavelength of 12 .mu.m and Hg.sub.0.73 Cd.sub.0.27 Te a bandgap of about 0.24 eV corresponding to a photon wavelength of 5 .mu.m; and these two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors. In particular, extrinsic p-type Hg.sub.1-x Cd.sub.x Te has potential application in infrared focal plane arrays operating in the 10-12 .mu.m wavelength window. (Intrinsic p-type Hg.sub.1-x Cd.sub.x Te, whose doping is presumably dominated by mercury vacancies, was recently found to have midgap recombination centers proportional in concentration to the shallow acceptors; see C. Jones et al, 3 J. Vac. Sci. Tech. A 131 (1985). These recombination centers shorten minority carrier lifetimes and are sources of recombination-generation noise; thus extrinsic p-type Hg.sub.1-x Cd.sub.x Te is preferred to intrinsic p-type. p Reproducible preparation of extrinsically doped p-type Hg.sub.1-x Cd.sub.x Te with carrier concentrations below about 1.times.10.sup.15 /cm.sup.3 is difficult. In fact, fast diffusing acceptor dopants (or impurities) preferentially segregate to regions containing precipitated tellurium. These regions of precipitated tellurium are normally at the core (central region) of a Hg.sub.1-x Cd.sub.x Te ingot or slice which arises during the usual manufacture of an ingot. The usual manufacture includes recrystallization and homogenization at a high temperature (650.degree. C.) followed by a low temperature (.ltoreq.300.degree. C.) anneal in mercury vapor for extended times to reduce the concentration of metal vacancies; this processing yields an n-type skin free of excess tellurium and a p-type core of condensed metal vacancies and precipitated tellurium. Fast diffusing residual impurities in the ingot or slice have been found to be gettered out of the skin region into the remaining core during the low temperature anneal; but when the core is annihilated by further low temperature anneal (up to several weeks duration), the impurities are released and redistribute throughout the ingot or slice and provide a p-type doping. See, H. Schaake et al, The Effect of Low Temperature Annealing on Defects, Impurities, and Electrical Properties of (Hg,Cd)Te, 3 J. Vac. Sci. Tech. A 143 (1985). Thus the slowness of the core-annihilating low temperature anneal and the stability of fast diffusing impurities (such as Group IB elements copper, silver, and gold) for doping are problems in the known methods of manufacture.
Further, methods of directly adding dopant material to a compounding ampoule containing mercury, cadmium, and tellurium only provide doping control for dopant concentrations of about 1.times.10.sup.17 /cm.sup.3 and greater. U.S. Pat. No. 4,462,959 provides a method of compounding doped mercury with cadmium and tellurium which can yield doping concentrations down to 1.times.10.sup.15, but these direct compounding methods are prone to impurities getting in the core and later redistributing as above described. In fact, copper diffusing through the quartz compounding ampoule apparently limits present efforts to control doping with Group IB elements; see J. Tregilgas et al, Type Conversion of (Hg,Cd)Te Induced by the Redistribution of Residual Acceptor Impurities, 3 J. Vac. Sci. Tech. A 150 (1985). Thus there are problems to control doping with fast diffusing dopants in Hg.sub.1-x Cd.sub.x Te and to rapidly annihilate the core of Hg.sub.1-x Cd.sub.x Te.